M.R. Bermejo

Use of electrochemical techniques for the study of solubilization processes of cerium-oxide compounds and recovery of the metal from molten chlorides

Abstract This work presents a study on the chemical and electrochemical properties of CeCl3 in two molten chloride mixtures with different oxoacidity properties, the eutectic LiCl–KCl at 723 K and the equimolar CaCl2–NaCl melt at 823 K. The E–pO2− (potential–acidity) diagram for Ce–O compounds stable in both melts has been constructed by combining both theoretical and experimental data. The stable oxidation states of Ce have been found to be (III) and (0) in both melts; Ce(IV) is only stable in the form of solid CeO2. The standard potential value of the Ce(III)/Ce(0) system has been determined by potentiometry, giving values −3.154±0.006 V and −3.036±0.009 V versus Cl2/Cl− in the eutectic LiCl–KCl at 723 K and the equimolar CaCl2–NaCl melt at 823 K, respectively. On the other hand, from the calculated activity coefficient values it was possible to say that the Ce(III) ions form stronger complexes with the chloride ions in the alkaline melt. This is probably due to the smaller amount of free chloride ions in the calcium molten mixture. Identification of the Ce–O compounds that are stable in the melts as well as the determination of their solubility products was easily carried out by potentiometric titration using an yttria stabilised zirconia membrane electrode (YSZME). The results indicated that Ce2O3 is a strong oxobase and that CeOCl is a solid stable compound in the melts studied. CeO2 is also a stable compound that can exist under oxobasic conditions. The best chlorinating conditions could be extracted from the comparison of the E–pO2− diagram corresponding to the Ce–O compounds and that of some chlorinating mixtures. Experimental solubilization tests allowed us to verify the thermodynamic chlorinating predictions. Moreover the electrochemical behaviour of CeCl3 solutions was also investigated. Transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry and chronoamperometry were used in order to study the reaction mechanism and the transport parameters of electroactive species on metallic substrates such as tungsten and molybdenum. The results showed that the Ce(III)/Ce(0) system is quasi-reversible with values of the charge transfer rate constant, k°, and transfer coefficient, α, around 10−3.7 cm s−1 and 0.4, respectively. The diffusion coefficient of Ce(III) ions was also determined by different electrochemical techniques, obtaining a value in the order of 1×10−5 cm2 s−1. The validity of the Arrhenius law was also verified by plotting the variation of the logarithm of the diffusion coefficient versus 1/T.

Chemical and electrochemical behaviour of chromium in molten chlorides

The chemical and electrochemical behaviour of chromium ions in the equimolar CaCl2 NaCl mixture at 550°C has been studied by electrochemical techniques (i.e. cyclic and square wave voltammetry, chronopotentiometry, chronoamperometry and potentiometry) using various substrates (Mo, W and glassy carbon) as working electrodes. Some of the results have been compared to data obtained in the eutectic LiClKCl mixture at 450°C. The stable oxidation states of chromium species were found to be the same in both melts. However, in the case of the CaCl2 NaCl melt and when using W substrates, a new electrochemical wave appears, which has been related to an enhancement of the oxidation of trace amounts of O 2 ions on the W material by the presence of Cr ions. Kinetic parameters characterising the mass and charge transfer of the electrochemical reduction and oxidation of Cr(II) ions as well as of the Cr(III) reduction process were also obtained. Chronoamperometric studies did not show evidence of nucleation phenomena in the case of the equimolar CaCl2 NaCl mixture when using any of the substrates. However, in the eutectic LiClKCl it has been shown that nucleation of metallic chromium, especially on glassy carbon substrates, controls the electroreduction process. The electrochemical behaviour of some chromium oxo-anions, i.e. Cr2O7 and CrO4 , has also been studied. The results show that both complexes lead to the same stable Cr(VI) species in the melt, whose reduction at glassy carbon electrodes implies a single one-electron process giving Cr(V) ions which are present in the melt as soluble species. Potentiometric titration of both Cr(III) and Cr(II) solutions were performed by using an oxide selective ion electrode, which consist of a membrane of ZrO2 stabilised with yttrium oxide (YSZME). The values of pKs for the different CrO compounds stable in the melt, i.e. CrO, Cr2O 2 and Cr2O3, were obtained.